This invention relates to an impregnated cathode for use in an electron tube such as a display tube, a picture tube, a pick-up tube, a traveling wave tube (TWT), etc. as a high current density cathode, and particularly to an impregnated cathode with higher electron emission.
The impregnated cathode is a high current density cathode and is promising as a cathode for higher quality, particularly higher resolution and higher brightness of en electron tube.
The conventional impregnated cathode has such a basic structure that a refractory porous body composed of W, etc. is impregnated with an electron emissive material composed of a barium (Ba) compound, and has a high electron emission property, but its operating temperature necessary for obtaining the necessary current density of 10 A/cm.sup.2 for the higher quality is as high as 1,100.degree.-1,200.degree. C., which is by about 400.degree. C. higher than that of the spray type oxide cathode now generally used. Owing to the high operating temperature, the electrode material must be a high melting point metal when it is practically used in an electron tube, and furthermore, a large amount of Ba and BaO (barium oxide) evaporates from the cathode and deposits onto the electrode, causing a grid emission and bringing an adverse effect on the electron tube characteristics. Furthermore, it is very difficult to design and produce a reliable heater capable of heating the impregnated cathode for a long duration. Thus, it is the most important task in the research and development of an impregnated cathode to lower the operating temperature of the impregnated cathode. In order to lower the operating temperature, the electron emission must be increased, and as a result the operating temperature will be lowered. According to a procedure for lowering the operating temperature, that is, a procedure for increasing the electron emission, as disclosed in Japanese Patent Publication No. 47-21343, the work function of the cathode is lowered by coating the cathode surface with a metal having a high work function such as an osmium (Os) -ruthenium (Ru) alloy, etc., thereby enhancing the electron emission, where the operating temperature of the impregnated cathode can be lowered by about 100.degree.-150.degree. C., which is still by 250.degree. C. higher than the operating temperature of the spray-type oxide cathode. Some of the present inventors thus proposed an impregnated cathode provided with a thin film composed of a high melting point metal and at least one of Sc and an oxide of Sc on the electron emissive surface of a refractory porous body [Japanese Patent Application Kokai (Laid-open) No. 51-13526]. The operating temperature of the cathode could be made lower by 100.degree.-150.degree. C. than that of the impregnated cathode coated with the said Os-Ru alloy, but the cathode had a little shorter life.